1. Field of the Invention
2. Description of the Related Art
In general, in vehicles such as motor cars equipped with an electric power steering apparatus, the steering torque applied to a steering shaft by an operator's steering effort on a steering wheel is detected by a torque sensor so that a control unit can control the current supplied to an electric motor operatively connected with the steering shaft in accordance with the steering torque thus detected, thus causing the electric motor to output a required amount of steering assist torque to the steering shaft through a speed reducer.
FIG. 12 is a block diagram of an electric power steering apparatus which uses a brushless motor as an electric motor.
This electric power steering apparatus includes an electric motor 1 which outputs assist torque to the steering wheel (not shown) of a vehicle, a control unit 6 which controls the operation of the electric motor 1, a battery 4 which supplies current to drive the electric motor 1, a torque sensor 5 which detects the steering torque of the steering wheel, a motor connector 15 which electrically connects the control unit 6 to the electric motor 1, a power connector 16 which electrically connects the battery 4 with the control unit 6, and a signal connector 17 which provides electrical connection between the electric motor 1, the torque sensor 5 and the control unit 6.
The electric motor 1 has an armature winding 2 which is connected in three phases to a stator (not shown) and a rotational position sensor 3 for detecting the rotational position of a rotor (not shown).
The control unit 6 includes three capacitors 7 of large capacities (e.g., about 2200 μF×3) for absorbing ripple components of a motor current IM flowing through the electric motor 1, a pair of shunt resistors 8 for detecting the motor current IM, a three-phase bridge circuit 10 which is composed of a plurality of semiconductor switching elements (for instance, FETs) Q1-Q6 for switching the motor current IM according to the magnitude and direction of the assist torque, a coil 11 for removing electromagnetic noise, and a control circuit 12.
The control circuit 12 includes a current detector 9 connected across the serially connected shunt resistors 8 for detecting a current flowing through the electric motor 1, and a microcomputer 13 for calculating the assist torque generated by the electric motor 1 based on a steering torque signal from the torque sensor 5, the microcomputer 13 also calculating the current corresponding to the assist torque by feeding back the motor current IM and the rotational position of the rotor detected by the rotational position sensor 3. The microcomputer 13 outputs a drive signal to the bridge circuit 10 through a drive circuit 14 so as to control the bridge circuit 10.
In addition, though not shown, the microcomputer 13 includes, in addition to an AD converter, a PWM timer circuit and the like, a well-known self-diagnosis function of self diagnosing at all times whether the system is working normally and of interrupting the motor current IM when there takes place abnormality.
FIG. 13 is a cross sectional plan view of the essential portions of the electric power steering apparatus with the circuit structure shown in FIG. 12. Here, note that in order to avoid the complexity of the drawing, only major circuit elements are illustrated while omitting peripheral circuit elements, wiring patterns, conductive wires, etc.
In this figure, an insulated printed-circuit board 19 is mounted on the bottom of a box-shaped metal frame 18 which has the functions of a shield board and a heat sink concurrently. For instance, a heat sink 20 made of aluminum is attached to one end face of the inner surface of the metal frame 18.
The capacitors 7, the shunt resistors 8, the coil 11, the microcomputer 13, etc., are mounted on the insulated printed-circuit board 19. Also, on the insulated printed-circuit board 19, there are arranged a plurality of conductive plates 21a-21e each having a large width and a large thickness besides the above-mentioned wiring patterns. In addition, the semiconductor switching elements Q1-Q6 are fixedly attached to one end face of the heat sink 20.
Now, the operation of the electric power steering apparatus as constructed above will be described below.
The microcomputer 13 receives an output signal from the torque sensor 5 representative of the steering torque, an output signal from the rotational position sensor 3 representative of the rotational position of the rotor of the electric motor 1, and the motor current IM fed back thereto from the shunt resistors 8 through the current detector 9, and generates a rotational direction instruction for power steering and a current control quantity corresponding to the assist torque, which are input to the drive circuit 14. When the rotational direction instruction and the current control quantity are input from the microcomputer 13, the drive circuit 14 generates a PWM drive signal and supplies it to the semiconductor switching elements Q1-Q6 of the bridge circuit 10. As a result, a current flows from the battery 4 to the electric motor 1 through external wiring, the power connector 16, the coil 11, the bridge circuit 10, the motor connector 15 and another external wiring, so that the electric motor 1 generates a required quantity of assist torque in a required direction.
At this time, the motor current IM flowing through the electric motor 1 is detected by means of the shunt resistors 8 and the current detector 9, and fed back to the microcomputer 13 so that it is controlled to be equal to a motor current instruction Im. In addition, the motor current IM contains ripple components which would be generated by the switching operation of the bridge circuit 10 when the bridge circuit 10 is driven to operate in a PWM manner, but it is controlled to be smoothed by means of the large-capacity capacitors 7. Moreover, the coil 11 serves to prevent the noise, which would be generated by the above-mentioned switching of the bridge circuit 10 during the PWM operation thereof, from being radiated outside to cause radio noise.
In the electric power steering apparatus as described above, the value of the motor current IM to be controlled is about 25-30 A in case of light cars but it reaches such a value as high as about 60-80 A in compact cars.
The control unit 6 of this electric power steering apparatus is installed in the passenger's compartment of an excellent environment in the vehicle, which is remote from the engine room thereof where the electric motor 1 and the battery 4 are arranged.
Accordingly, a plurality of long wires or cables are required for providing electrical connection between the control unit 6 and the electric motor 1. As a result, there arises a problem that the cost of manufacture becomes high and the weight of the vehicle increases.
In addition, if the length of wiring is extended, there takes place another problem in that a power loss is increased and radiation noise generated due to the PWM operation of the bridge circuit 10 is increased as well. Also, the increased radiation noise might cause a malfunction of other control equipment, radio noise, etc.
Furthermore, the size of the semiconductor switching elements Q1-Q6 constituting the bridge circuit 10 in the control unit 6 increases in accordance with the increasing magnitude of the motor current IM. It is also necessary to control the generation of heat at the time of the plurality of semiconductor switching elements Q1-Q6 being turned on or subjected to the PWM switching operation by arranging them in parallel with each other. Thus, the size of the heat sink 20 has to be increased to improve the radiation of heat from the semiconductor switching elements Q1-Q6. Moreover, the sizes of the respective electronic members employed in various parts of the apparatus increase in proportion to the increasing motor current IM, and hence it is required to physically or practically increase the length of a wiring pattern extending from terminals of the power connector 16 to ground by way of the coil 11, the bridge circuit 10 and the shunt resistors 8 as well as the length of a wiring pattern extending from the bridge circuit 10 to the motor connector 15.
As a result of the various reasons as referred to above, there arise the following additional problems. That is, the size of the control unit 6 is necessarily enlarged; the magnitude of the motor current IM is decreased by voltage drops in the respective wiring patterns; and the durability of the wiring patterns is reduced by the heat generated.